1. Field of the Invention
The present invention relates to a D/A (Digital-to-Analog) conversion apparatus and a D/A conversion method that are suitable for, for example, an electric musical instrument or the like, and an electric musical instrument with the D/A conversion apparatus.
2. Description of the Related Art
A D/A conversion apparatus is conventionally known which uses a delta-sigma modulator that produces a noise shaping effect of shifting quantization noise to a high-pass side so as to improve a S/N (Signal-to-Noise) ratio in an audible band. As this type of apparatus, Japanese Patent No. 4888837 discloses a D/A conversion apparatus including a delta-sigma modulator which performs delta-sigma modulation on an input signal according to a first clock and generates a delta-sigma modulated output; a timing generator which generates a start signal indicating the start of a time slot assigned as the processing time of the delta-sigma modulator per sample based on the first clock and a second clock that is asynchronous with the first clock, and generates a completion signal indicating the completion of delta-sigma modulation processing by the delta-sigma modulator; and an output conversion section which converts the delta-sigma modulated output generated by the delta-sigma modulator in synchronization with the first clock into an output signal (PWM—Pulse Width Modulation) output signal) synchronized with the second clock, according to the start signal and the completion signal generated by the timing generator. This D/A conversion apparatus can be easily mounted in a high-order system, such as ASIC (Application Specific Integrated Circuit) and FPGA (Field Programmable Gate Array), and can avoid the effect of a jitter (time-base error) without requiring a complicated apparatus structure.
However, this D/A conversion apparatus disclosed in Japanese Patent No. 4888837, which uses delta-sigma modulation with PWM (Pulse Width Modulation), has the following problem. Hereafter, this problem will be described with reference to FIG. 7A to FIG. 9.
Normally, in the interface of a D/A conversion apparatus constituted by a digital circuit and an analog circuit section used as the subsequent stage, a buffer that serves as an output element is commonly provided, whereby a PWM output signal outputted from the D/A conversion apparatus is supplied to the analog circuit section used as the subsequent stage.
In the buffer which drives a PWM output signal, a drive characteristic of drawing an electric current and performing pull-down to the “L” level from the “H” level as shown in FIG. 7A and a drive characteristic of emitting an electric current and performing pull-up to the “H” level from the “L” level as shown in FIG. 7B are different. That is, in the buffer, the drive capability to transition from the “H” level to the “L” level and the drive capability to transition from the “L” level to the “H” level are different.
In order for the D/A conversion apparatus to operate ideally, the amount of current to be “drawn” (a shaded area “a” in FIG. 8) and the amount of current to be emitted (a shaded area “b” in FIG. 8) should preferably be the same irrespective of the voltage before the transition, as shown in FIG. 8. However, when the pull-down and pull-up characteristics (drive capabilities) differ from each other as described above, a delay time Δt1 when transition is performed from “L” level to “H” level and a delay time Δt2 when transition is performed from “H” level to “L” level differ as shown in FIG. 9, which causes a duty error of a PWM output signal to occur. As a result, the conversion accuracy is degraded.